This invention relates to a projecting optical system having a light beam deflector.
There has heretofore been known an optical system as shown in FIG. 1 which is one example of an optical system to project a light beam on a second plane, while scanning a first plane with a light beam deflector. In the optical system shown in FIG. 1, a reference numeral 1 designates a first plane, and numeral 2 refers to a so-called f.multidot.tan .theta. lens having one of its focal points on the first plane and being free from distortion. A numeral 3 refers to a deflector, in accordance with rotation of which the first plane is scanned in the direction of an axis Y. A numeral 4 also designates an f.multidot.tan .theta. lens which is so disposed that one of its focal planes may coincide with the second plane 5. Accordingly, the light beam of the plane 1 is rendered a parallel light beam by the f.multidot.tan .theta. lens 2 and this parallel light beam is deflected by the deflector 3, after which it is focused on the second plane 5 by the f.multidot.tan .theta. lens 4 in the same manner as above. A lens system, in which the first plane and the second plane are made conjugative by such layout, is called a `tandem` lens system. As has been well known, the purpose of this f.multidot.tan .theta. lens is to focus the parallel light beam entering at an incident angle of .theta. with respect to the optical axis of the lens in such a manner that a distance y from the optical axis may satisfy the following relationship on its focal plane: y=f.multidot.tan .theta.. In case the deflector 3 is interposed between these f.multidot.tan .theta. lenses 2 and 4 constituting the tandem system, an image component in the X direction which orthogonally intersects with the principal scanning direction Y on the first plane 1 can be focused on the second plane free from distortion.
In the abovementioned projecting optical system, when it is desired to impart a desired deflecting angular speed characteristic to the deflector, or when it is desired to impart to it a desired scanning speed in the principal scanning direction Y of the first plane, it is necessary to use as the lens constituting the tandem system, a lens having a distortional characteristic in place of an f.multidot.tan .theta. lens having no distortional characteristic. However, when the lens having the distortional characteristic is replaced by the f.multidot.tan .theta. lens (2, 4) in the projecting device shown in FIG. 1, the image component in the X direction which orthogonally intersects with the principal scanning direction Y of the first plane 1 to be projected on the second plane 5 accompanies the distortion.